How can you tell if two people are in love?

More eye contact:

This study reports the initial results of an attempt to introduce and validate a social-psychological construct of romantic love. Starting with the assumption that love is an interpersonal attitude, an internally consistent paper-and-pencil love scale was developed. The conception of romantic love included three components, affiliative and dependent need, a predisposition to help, and an orientation of exclusiveness and absorption. Love-scale scores were only moderately correlated with scores on a parallel scale of “liking,” which reflected a more traditional conception of interpersonal attraction. The validity of the love scale was assessed in a questionnaire study and a laboratory experiment. On the basis of the emerging conception of love, it was predicted that college dating couples who love each other a great deal (as categorized by their love-scale scores) would spend more time gazing into one another’s eyes than would couples who loved each other to a lesser degree. The prediction was confirmed.

According to Rubin, normally two people in conversation give each other eye contact anywhere from 30-60% of the time but couples who are in love look at each other 75% of the time during conversation and are slower to break their look away from each other when interrupted.